I have a Craftsman Model # 917.258552 lawn tractor with a kohler engine and a hydrostatic transmission. My problem is that after running for about 15 minutes the trnsmission seems to lose power. It starts out gradually and gets to the point where it will not move the tractor at all. I have noticed it is worse if you are on an incline. if you are on level ground the problem does still happen but not as bad. I have purged the transmission, but it does not seem to help. I checked the fluid and it is full. I also checked the transaxle pulley and it is spinning when I have the transmission engaged, but the tractor is still not moving. Any help would be greatly appreciated.